Many coal-fired power plant boilers are designed for tangential firing, i.e., a configuration in which streams of pulverized coal and air are directed into a rectangular furnace compartment from columns of nozzles located in such a way as to generate a slowly rotating cyclonic fireball, which produces heat which in turn boils water in arrays of water tubes lining the walls of the compartment. Tangential firing is described in various patents including U.S. Pat. Nos. 4,252,069, 4,634,054, and 5,483,906.
Tangentially fired boilers fueled by pulverized coal typically have pivotable coal nozzle tips protruding into the furnace. Biomass fuel nozzles are similar to these coal nozzles. The coal nozzle tips have a double shell configuration, comprising an outer shell and an inner shell. The inner shell is coaxially disposed within the outer shell to provide an annular space between the inner and outer shells. The inner shell is connected to a fuel feeding conduit or pipe for feeding pulverized coal, entrained in air flowing through the inner shell, into the furnace. The annular space between the inner and outer shells is connected to a secondary air conduit for feeding secondary air into the furnace. The secondary air not only serves as supplemental combustion air, but also cools the inner and outer shells. The fuel feeding pipe is typically disposed coaxially within in the secondary air conduit.
A furnace will typically have not only several coal nozzle tips at each corner, but also several air nozzle tips, arranged in a column along with the coal nozzle tips, to introduce additional secondary air into the furnace.
The nozzle tips, which are typically made from stainless steel plate having a thickness from ¼ to ¾ inch, are located in an opening in a nozzle supporting wall, typically in the outlet of the secondary air box. The external cross section of a nozzle tip is typically rectangular, and corresponds to the internal cross section of the outlet end of the air conduit. Narrow gaps between the peripheral walls of the nozzle tip and the walls of the air conduit allow leakage of secondary air into the furnace. When the nozzle tips discharge air, or fuel and air, horizontally into the furnace, the air leaking through these gaps flows along the external walls of the nozzle tips and normally prevents the nozzle tip from being heated excessively by radiation from the fire ball within the furnace.
In a typical tangentially fired burner, the nozzle tips are pivotable upward and downward so that the position of the fire ball can be controlled. When a nozzle tip is tilted to provide an upward or downward flow of air, or fuel and air, into the furnace, one of its walls will be bent away from the air flow leaking through the gap between that wall and an adjacent wall of the air conduit, and the protection afforded that wall by leaking air will be greatly diminished.
Unprotected exposure to radiation when the nozzle tips are tilted upward or downward, induces thermal gradients in the thick stainless steel. The thermal gradients cause distortion of the nozzle tips, and can even cause eventual closure of their air and fuel passages. Unprotected exposure to radiation also results in excessively high temperatures, oxidation, and thinning of the stainless steel plate. Thermal distortion and high temperature oxidation of the nozzle tips cause heavy damage to the nozzle tips and deterioration of combustion performance, requiring frequent and expensive replacement. Similar problems are encountered in the case of nozzle tips mounted for yaw adjustment or for both pitch and yaw adjustment.
In U.S. Pat. No. 6,260,491, I describe a tiltable nozzle tip that addresses the problem of excessive heating by directing air over the front part of the outer shell of the nozzle tip from a channel formed between a rear part of the outer shell and an external shroud provided on the nozzle tip. The air flows from the channel along the front part of the outer shell even when the nozzle tip is tilted, and thereby protects the nozzle tip from distortion and failure due to excessive heat.
Although the air-directing channels described in U.S. Pat. No. 6,260,491 are effective to reduce thermal distortion and high temperature oxidation of a nozzle tip, even a nozzle tip equipped with such air-directing channels is subject to eventual failure due to thermal distortion and oxidation when exposed to radiation and hot gases in a furnace over an extended time.
A problem inherent in conventional fuel and air nozzle tips, as well as in nozzle tips equipped with air-directing channels, is that the outside surfaces of the nozzle tips are exposed to high temperatures due to flame radiation, conduction of heat from hot gases, or a combination of radiation and conduction, while the fuel, air, or a combination of fuel and air passing through the inside of the nozzle is relatively cool, and tends to cool the inside surfaces of the nozzle. The difference between the temperature of the outside surfaces and the temperature of the inside surfaces results in a high temperature gradient across the plates or castings that make up the nozzle. When one side of a plate or casting is cooled while the other side becomes very hot due to furnace radiation, hot gas, or both, the plate or casting distorts, and the structural integrity of the nozzle is compromised. The nozzle becomes less effective for its intended purpose, and its service life is shortened.